Single Atom X-ray Spectroscopy using Synchrotron X-rays Scanning Tunneling Microscopy*

X-rays are produced by excitations of the core level electrons in atoms, and they are useful to detect the type of elements in the periodic table. However, X-ray characterization generally requires a large number of atoms to attain a detectable signal and reducing the size of a sample for X-ray experiments is a long-time goal. To date, X-ray detection of materials can be made on the samples with as few as ~10⁴ atoms due to advances in instrumentation. Here, we show that X-rays can be used to detect metal atoms at the ultimate atomic limit in a quantum tunnelling regime: A single iron atom caged in an organic host, a hexagonal shape supramolecule, has been detected using synchrotron X-ray scanning tunneling microscopy. The experiments were conducted at the XTIP beamline [1] located at the sector 4-ID-E of the Advanced Photon Source of Argonne National Laboratory. Using a specialized coaxial tip positioned at the extreme proximity to the molecule as a detector, the photocurrent generated from the synchrotron X-ray excitation of the iron atom is recorded. The fingerprints of the iron atom, the L₃ and L₂ edge signals at 708.9 eV and 722.1 eV energies originating from the 2p 3/2 and 2p ½ to ‘d’ transitions, are directly observed in the X-ray absorption spectra.

[1] V. Rose, N. Shirato, M. Bartlein, A. Deriy, T. Ajayi, D. Rosenmann, S.-W. Hla, M. Fisher, and R. Reininger. XTIP – the world's first beamline dedicated to the synchrotron X-ray scanning tunneling microscopy technique. J. Synchrotron Rad. 27, 836-843 (2020).